The invention relates to media signal processing (e.g., image, audio and video), and specifically relates to a method for removing noise from media signals.
Processing of media signals often introduces noise that impacts signal fidelity and interferes with communication. For example, in digital imaging, image capture and printing processes introduce noise into images. Similarly, converting audio to and from the digital domain introduces noise. Fixed pattern noise is a form of noise that appears as a fixed pattern in a media signal. For example, an image sensor or audio transducer may introduce a pattern of noise that is common throughout the signals captured in the device. For many signal processing applications, there is a need for effective methods for removing fixed pattern noise and other fixed patterns. One such application is digital watermarking, but there are many others.
Digital watermarking is a process for modifying media content to embed a machine-readable code into the data content. The data may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media such as images, audio signals, and video signals. However, it may also be applied to other types of data, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems have two primary components: an encoder that embeds the watermark in the media content, and a decoder that detects and reads the embedded watermark. The encoder embeds a watermark by altering data samples of the media content. The reading component analyzes content to detect whether a watermark is present. In applications where the watermark encodes information, the reader extracts this information from the detected watermark.
The invention provides a method of removing fixed pattern noise from a media signal. The method derives an estimate of fixed pattern noise from a composite media signal and uses the estimate to evaluate and remove fixed pattern noise from selected frames. The technique is particularly suited for removing fixed pattern noise in images due to image capture processes, but applies to other media signals as well. In watermarking applications, the method can be used to improve detection and recovery of a watermark by removing a watermark or watermark component that might otherwise interfere with watermark detection or reading. In particular, by removing one watermark, the method may enhance detection and reading of another watermark.
The method may also be used to remove a watermark so that another can be added in transcoding a watermarked media signal from one watermark format to another. In this application, a watermark removal method estimates and removes a first watermark. Then, a second and potentially different watermark is encoded in the media signal.
One implementation of the method combines frames of the media signal to form a composite signal. It then removes a DC component from the composite signal and derives a measure of fixed pattern noise from the composite signal. Using the measure of fixed pattern noise, it computes an estimate of fixed pattern noise in a frame of the media signal. Finally, it removes the estimate of fixed pattern noise from the frame.
The method may be implemented in software or hardware and may be used in a variety of applications. In one application, it is implemented in software and operates as a watermark decoder preprocessor. The preprocessor includes a compositer, filter, noise estimater, and noise remover. The compositer combines frames of a media signal into a composite signal. The filter, which may include one or more stages, removes a DC component from the composite signal and creates a measure of fixed pattern noise. Operating on selected frames, the noise estimater uses the measure of fixed pattern noise to estimate fixed pattern noise in a selected frame of the media signal. The noise remover removes the estimate of the fixed pattern noise from the selected frame.
Further features of the invention will become apparent from the following detailed description and accompanying drawings.